It is common for commercial and agricultural vehicles, such as tractors and harvesters, to use a vertical stack pipe for the engine exhaust. It is also known to provide sensors, such as NOx or lambda sensors, within vehicle exhaust pipes to monitor the contents of the exhaust gases, the output signals of such sensors being used by the engine management system. A problem that arises when an exhaust stack pipe is fitted with a sensor is that water ingress can damage the sensor caused for instance by the large temperature difference between the water and the sensor in working conditions. Water ingress may further also generate a false read-out of a sensor. Steps need to be taken to prevent rain or other precipitation from penetrating into the stack pipe.
Hitherto, this has been achieved by placing a flat cover over the end of the stack pipe. Such an arrangement is described, for example, in U.S. Pat. No. 7,347,044 in the context of a vertical exhaust water trap assembly having an outer housing and an internal housing or exhaust tube defining an annular water collection spaced therebetween bypassing a central exhaust flow area. However, such an obstruction in the path of the exhaust gases creates a backpressure within the exhaust system which reduces engine performance and efficiency. An increased backpressure is critical for vehicles equipped with an exhaust aftertreatment system. Due to the reduced speed of the exhaust gases the temperature in these aftertreatment systems increase and components may be damaged permanently, resulting in high warranty and maintenance costs.